(Invited) Highly Active and Robust Pt-Skin/Pt Alloy Two-Way Catalysts for Oxygen Reduction and Hydrogen Oxidation in PEFCs

Abstract

Cathode catalysts for the oxygen reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) have been investigated toward increasing their activity and durability, which can promote the large-scale commercialization of fuel cell vehicles (FCVs) and residential co-generation systems. In contrast, anode catalysis R&D for the hydrogen oxidation reaction (HOR) has not been so intensive recently. One of the reasons is that the HOR rate is sufficiently fast at Pt/C anode catalysts with a low Pt loading (ca. 0.05 mgPt cm−2) for FCVs with pure H2 as the fuel. For residential PEFCs operated with reformate (H2-rich fuel gas), Pt-Ru anode catalysts have been used predominantly to mitigate the poisoning by CO (below 50 ppm). However, Ru is not only costly (mass activity for the HOR at Pt-Ru/C is low) but is also unstable at high potentials (E > 0.8 V). We demonstrated that both bulk Pt-Co and commercial c-Pt3Co/C exhibited high CO-tolerance in 0.1 M HClO4 solution at temperatures below 70 °C.1 It has been clarified that the Co component leached out into the acidic solution, resulting in the formation of a thin Pt-skin layer on the alloy surface. The excellent CO-tolerance on the Pt-skin layer was ascribed to weaker CO adsorption, which can maintain active sites for the HOR, due to its electronic structure being modified by the underlying alloy.1 However, the CO-tolerance was lost after being heated ≥ 80 °C due to increased dealloying of the Co component, leading to a thick Pt layer. Such dealloying of Co from c-Pt3Co/C catalysts during load-change cycles is also a serious issue. Very recently, we have succeeded in suppressing the dealloying of Co by forming a thin, uniform, stabilized Pt-skin (one to two atomic layers: xAL) on Pt-Co alloy nanoparticles (PtxAL–PtCo), which act as a two-way catalyst for the ORR cathode with high activity and durability2,3 and the HOR anode with high activity, CO tolerance, and robustness at 70 and 90 °C.4 This presentation focuses on the following topics concerning the unique characteristics of the stabilized Pt-skin catalysts, including the analyses of microstructure and electronic structure. 1) Important role of Pt-skin layer and underlying alloy for enhancement of the ORR activity5, 6 The j k value for the ORR at 0.9 V on Pt-skin/Pt100−xCox(111) single crystals reached a maximum at x = 27 atom% that was ca. 27 times higher than that on a pure Pt(111) electrode.5 By the use of in situ STM, in situ SXS, ex situ angle-resolved XPS, and DFT calculations, we have succeeded in correlating this high j k with the specific surface structure and electronic structure.6 2) Potential-cycle induced change in crystal structure of PtxAL–PtCo/CWe have found that the crystal structure of PtxAL‒PtCo nanoparticles [face-centered tetragonal (fct) + face-centered cubic (fcc)] was converted to single-phase fcc, which exhibited high ORR activity and durability, surprisingly, only by means of potential cycling from 0.05 V to 1.0 V in 0.1 M HClO4 solution at room temperature. 3) Multilateral analyses of the enhancement of HOR activity and CO-tolerance for the PtxAL‒PtM/C7, 8 It was found by in situ FTIR,7 in situ XAS,8 and DFT calculations8 that the adsorption of CO on terrace sites on the PtxAL skin covering the PtM alloy (M= Fe, Co) was weakened by the modified electronic structure. This work was supported by funds for the ‘‘Superlative, Stable, and Scalable Performance Fuel Cell (SPer-FC)’’ project and “High Performance Fuel Cell (HiPer-FC)” project from NEDO of Japan. References H. Uchida, K. Izumi, K. Aoki, and M. Watanabe, Phys. Chem. Chem. Phys., 11, 1771 (2009).M. Watanabe, H. Yano, D. A. Tryk, and H. Uchida, J. Electrochem. Soc., 163, F455 (2016).M. Chiwata, H. Yano, S. Ogawa, M. Watanabe, A. Iiyama, and H. Uchida, Electrochemistry, 84, 133 (2016).G. Y. Shi, H. Yano, D. A. Tryk, A. Iiyama, and H. Uchida, ACS Catal., 7, 267 (2016).S. Kobayashi, M. Wakisaka, D. A. Tryk, A. Iiyama, and H. Uchida, J. Phys. Chem. C, 121, 11234 (2017).S. Kobayashi, M. Aoki, M. Wakisaka,T. Kawamoto, R. Shirasaka, K. Suda, D. A. Tryk, J. Inukai, T. Kondo, and H. Uchida, ACS Omega, 3, 154 (2018).Y. Ogihara, H. Yano, T. Matsumoto, D. A. Tryk, A. Iiyama, and H. Uchida, Catalysts, 7, 8 (2017).G. Y. Shi, H. Yano, D. A. Tryk, M. Matsumoto, H. Tanida, M. Arao, H. Imai, J. Inukai, A. Iiyama, and H. Uchida, Catal. Sci. Technol., 7, 6124 (2017).